1. Field of the Invention
The present invention provides a continuous process for the oxidation of alcohols with the aid of nitroxyl compounds as catalysts in multiphase systems.
2. Background Art
The oxidation of alcohols to aldehydes or ketones is an important transformation in organic chemistry, since compounds having a high reaction potential are formed from readily available alcohols. Such transformations are therefore of great importance in industrial processes. Catalytic processes are particularly advantageous. A process frequently employed in industry is the gas-phase dehydrogenation of alcohols. However, only volatile compounds can be used in this process. The catalyst systems employed are useful with only a few substrates, and the reaction conditions have to be matched specifically to these substrates. Oxidations employing nitroxyl compounds, in particular TEMPO (2,2,6,6-tetramethylpiperidin-1-oxyl) and its derivatives (Review: A. E. J. de Nooy, A. C. Besemer and H. V. Bekkum, SYNTHESIS 1996, 1153), as oxidation catalysts are more generally applicable. Numerous TEMPO derivatives, including TEMPO derivatives on polymeric supports, e.g. polyamine immobilized piperidinyl oxyl xe2x80x9cPIPOxe2x80x9d, Dijksman et al., Synlett 2001, 102, EP 1103537, have been described as oxidation catalysts. These TEMPO-catalyzed oxidations are frequently carried out in two-phase systems, e.g. methylene chloride/water, as disclosed by P. L. Anelli, C. Biffi, F. Montanari and S. Quici, J. Org. Chem. 1987, 52, 2559. The oxidation of alcohols using sodium hypochlorite or sodium hypobromite as oxidant has been studied in detail. Aldehydes can be obtained from primary alcohols, and ketones from secondary alcohols by this process.
When the syntheses are, as is customary, carried out batchwise, the oxidant dissolved in the aqueous phase is added to the organic phase containing the alcohol to be oxidized and the nitroxyl compound. A disadvantage of this existing procedure is that the heat of reaction of the strongly exothermic nitroxyl-catalyzed oxidation process can be removed only with great difficulty, particularly in the case of large batches. This necessitates an increase in the contact time of the two phases and thus the duration of the process, as a result of which secondary reactions, e.g. reaction of alkali-labile groups such as ester group saponification, become significantly more prominent. For the purposes of the present invention, the contact time is the time over which the phases participating in the reaction are mixed.
A further secondary reaction which occurs in batch reactions involves formation of hemiacetals with the starting alcohol and further oxidation to the corresponding ester in accordance with equation 1. 
This reaction becomes increasingly prominent with increasing reaction time and phase contact time, so that the achievable yields decrease with increasing batch size, rendering industrial production impossible. A further secondary reaction which decreases the yield of the aldehyde is the further oxidation of the aldehyde to the corresponding carboxylic acid.
For example, the reaction of 20 g of 2-n-butyryloxyethanol formed only 37% of the desired aldehyde together with 12% of 2-n-butyryloxyethyl 2-n-butyryloxyacetate, 4% of butyric acid and 41% of 2-n-butyryloxyacetic acid, as shown in Comparative Example C9 herein. On doubling the batch size, the yield of aldehyde decreased to only 12% and the amount of 2-n-butyryloxyethyl 2-n-butyryloxyacetate was as high as 45% as shown in Comparative Example C10.
EP 0340703 and DE 4007923 describe TEMPO-catalyzed oxidation reactions using sodium hypochlorite solution in a batch reaction. The contact times are more than 15 minutes and particularly more than 30 minutes, since the post-reaction alone required these amounts of time. Although the possibility of carrying out this process continuously was considered, no continuous process was exemplified.
DE 10029597 describes an oxidation process employing a polymer-enlarged TEMPO derivative, likewise as a batch process. The possibility of carrying out the process continuously is envisaged only in conjunction with the use of a membrane reactor, but is not exemplified. Here too, contact times of at least 30 minutes are always employed. DE 69415345 (EP 0734392) employs contact times of 5 hours. In DE 19605039 (EP 0801073), a contact time of at least 45 minutes is employed. In EP 1103537, the after-reaction time alone is 20 minutes. In WO 01/90111, contact times as high as 4 hours were used, and 2 hours were involved with the after-reaction alone.
It is, therefore, an object of the invention to provide a process for the oxidation of an alcohol to an aldehyde or a ketone in the presence of a nitroxyl compound as catalyst, which process gives higher yields of aldehydes or ketones.
This and other objects are achieved by a process in which the alcohol to be oxidized in an organic liquid phase is reacted in the presence of a nitroxyl compound with an aqueous phase comprising the oxidant, wherein the reaction is carried out continuously at a contact time of the phases of from 0.1 s to a maximum of 15 minutes with intensive mixing of the phases.
It has been surprisingly discovered that simply converting the batch process into a continuous process does not improve the yields. Although no ester formation is then observed, the further oxidation of the aldehyde to the carboxylic acid occurs to an increased extent. The simple conversion of the batch process into a continuous process therefore does not represent a solution to the yield problems. This can also be seen from the Comparative Examples C11 and C12 of the present patent application.
In the process of the invention, the combination of the use of a continuous process together with contact times below 15 minutes is essential. The above-described secondary reactions are suppressed efficiently only by adherence to both process requirements.
The continuous reactors required for the process are known to those skilled in the art. An overview of the most important embodiments are given in, for example, xe2x80x9cUllmann""s Encyclopedia of Industrial Chemistryxe2x80x9d, Vol. B4. The process can be carried out, for example, in continuously operated tube reactors, in continuously operated loop reactors, in continuously operated stirred vessels or cascades of stirred vessels, or by means of centrifugal pumps. In a particularly preferred embodiment, the two phases are combined in a static mixing element and then directed through a tube reactor.
The contact time of the phases is preferably from 1 s to 5 minutes, more preferably from 1 s to 2 minutes, and most preferably from 1 s to 30 s.
Intensive mixing of the phases is preferably achieved by establishing turbulent flow in the reaction mixture. Particular preference is given to turbulent flow at a Reynolds number Re of from 800 to 20,000. Intensive mixing or turbulent flow can be achieved by means of all known mixing systems, e.g. static mixing elements or stirrers. Various mixing systems can also be combined to reach or exceed the Reynolds number.
The organic liquid phase comprises the alcohol and, if desired, one or more organic solvents. Suitable organic solvents include, for example, linear or branched saturated or unsaturated aliphatic hydrocarbons having 1-20 carbon atoms, cyclic aliphatic saturated or unsaturated hydrocarbons having 5-20 carbon atoms or aromatic hydrocarbons having 5-20 carbon atoms, in each of which one or more hydrogen atoms or one or more carbon atoms may be replaced by heteroatoms.
Preference is given to linear or branched saturated or unsaturated aliphatic hydrocarbons having 1-16 carbon atoms, cyclic aliphatic saturated or unsaturated hydrocarbons having 5-16 carbon atoms or aromatic hydrocarbons having 6-16 carbon atoms, in each of which one or more hydrogens may be replaced, independently of one another, by F, Cl, Br, NO2 or CN, or one or more CH2 groups may be replaced, independently of one another, by O, NH, Cxe2x95x90O, S, Sxe2x95x90O, SO2, Pxe2x95x90O, or one or more CH groups may be replaced, independently of one another, by N or P, or quaternary carbons can be replaced by Si.
Examples of suitable organic solvents include hexane, petroleum ether, cyclohexane, decalin, methylene chloride, chloroform, carbon tetrachloride, 1,2-dichloroethane, chlorobenzene, benzene, toluene, 1-chloronaphthalene, ethyl acetate, butyl acetate, ethylene carbonate, tetrahydro-1,3-dimethyl-2(1 H)-pyrimidinone (DMPU), hexamethylphosphortriamide (hexamethylphosphoramide, HMPA), dimethyl sulfoxide (DMSO), diethyl ether, methyl tert-butyl ether, ethylene glycol diethyl ether, diethylene glycol diethyl ether, dioxane, diisopropyl ketone and polydimethylsiloxanes.
The alcohol to be oxidized can be used in concentrations of from 0.1 to 100% by weight based on the organic solution, preferably from 1 to 50% by weight.
For the purposes of the present invention, the term nitroxyl compound encompasses both the oxidized form of the oxoammonium ion and the reduced form of the hydroxylamine, since these forms both occur in the oxidation (A. E. J. de Nooy, A. C. Besemer and H. V. Bekkum, Synthesis 1996, 1155).
The nitroxyl compound used as oxidation catalyst is preferably a di-tert-alkylnitroxyl compound, preferably a nitroxyl compound of the formula I: 
where the radicals R1, R2, R3 and R4 are each, independently of one another, C1-C12-alkyl, C2-C12-alkenyl, or C6-C12-aryl or aralkyl, and the radicals R5 and R6 each, independently, are hydrogen, OH, CN, halogen, linear or branched, saturated or unsaturated C1-C20-alkyl, C6-C20-aryl, C6-C20-hetaryl or C6-C20-aralkyl, OR7, Oxe2x80x94COR7, Oxe2x80x94COOR7, OCONHR7, COOH, COR7, COOR7, or CONHR7, where R7 is a linear or branched, saturated or unsaturated C1-C20-alkyl, C6-C20-aryl, C3-C20-hetaryl, or C6-C20-aralkyl radical, xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2)nxe2x80x94OR8, xe2x80x94(Oxe2x80x94C3H6)nxe2x80x94OR8, xe2x80x94(Oxe2x80x94(CH2)4)nxe2x80x94OR8, or xe2x80x94Oxe2x80x94CH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2xe2x80x94)nxe2x80x94OR8, where R8 is hydrogen, C1-C20-alkyl or C6-C20-aralkyl, and n=1 to 100, or CH2xe2x80x94CHOHxe2x80x94CH3, CH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94CH3, NR9R10, NHCOR9, NHCOOR9, or NHCONHR9, where R9 and R10 are each independently, a linear or branched, saturated or unsaturated C1-C20-alkyl, C6-C12-cycloalkyl, C6-C20-aryl, C3-C20-hetaryl, or C6-C20-aralkyl radical, where the radicals R5 and R6 may also be joined to form a ring, and the radicals R5 and R6 may in turn be substituted by COOH, OH, SO3H, CN, halogen, primary, secondary or tertiary amino or quaternary ammonium or the radicals R5 and R6 may together constitute xe2x95x90O, xe2x95x90NR11, xe2x95x90Nxe2x80x94OR11, or xe2x95x90Nxe2x80x94Nxe2x95x90CR11R12, where R11 and R12 are each independently, hydrogen, C1-C20-alkyl or C6-C20-aralkyl.
Preference is also given to nitroxyl compounds formed from two molecules of the formula I which are linked in the 4 position by a bridge xe2x95x90Nxe2x80x94Nxe2x95x90. Preference is further given to nitroxyl compounds formed from two or more molecules of the formula I joined to one another via one of the two radicals R5 or R6. Suitable linking groups are, for example, Oxe2x80x94alkylxe2x80x94O, Oxe2x80x94CH2-aryl-CH2xe2x80x94O, Oxe2x80x94COxe2x80x94NH-aryl-NHxe2x80x94COxe2x80x94O, Oxe2x80x94COxe2x80x94NH-alkyl-NHxe2x80x94COxe2x80x94O and bridges of the formula (Oxe2x80x94(CH2)nxe2x80x94O)m where n=2 to 4 and m=2 to 50.
In a further embodiment, the nitroxyl compound is a polymeric structure comprising compounds of the formula I which are linked via the radicals R5 or R6 or R5 and R6. The literature describes many such structures (EP 1103537, Cirriminna et al., Chem. Commun. 2000, 1441; Bolm et al., Chem. Commun. 1999, 1795; Bobbitt et al., Chem. Commun. 1996, 2745, Miyazawa and Endo, J. Molec. Catal. 49, 1988, L31; M. J. Verhoef et al. in xe2x80x9cStudies in Surface Science and Catalysisxe2x80x9d, Vol. 125, p. 465 ff; D. Brunel et al. in xe2x80x9cStudies in Surface Science and Catalysisxe2x80x9d, Vol. 125, p. 237 ff; Miyazawa and Endo, J. POLYMER SCI., Polym. Chem. Ed. 23, 1985, 1527 and 2487; T. Osa, Chem. Lett. 1988, 1423). Examples are PIPO, SiO2-supported TEMPO, and polystyrene- and polyacrylic acid-supported TEMPO.
A particularly preferred nitroxyl compound is a compound of the formula I in which R1, R2, R3 and R4 are each CH3 and R5 and R6 are each, independently, hydrogen, OH, OR7, Oxe2x80x94COR7, Oxe2x80x94COOR7, or OCONHR7, where R7 is a linear or branched, saturated or unsaturated C1-C20-alkyl radical or a C6-C20-aryl or C6-C20-aralkyl radical, xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2)nxe2x80x94OR8, xe2x80x94(Oxe2x80x94C3H6)nxe2x80x94OR8, xe2x80x94(Oxe2x80x94(CH2)4)nxe2x80x94OR8, or xe2x80x94Oxe2x80x94CH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2xe2x80x94)nxe2x80x94OR8, where R8 is hydrogen, C1-C10-alkyl or C6-C10-aralkyl, where n=1 to 100, CH2xe2x80x94CHOHxe2x80x94CH3, CH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94CH3, NR9R10, NHCOR10, NHCOOR10, or NHCONHR10, where R9 and R10 are each, independently, hydrogen, a linear or branched, saturated or unsaturated C1-C20-alkyl, C6-C12-cycloalkyl, C6-C20-aryl, or C6-C20-aralkyl radical.
The nitroxyl compound is more preferably a compound of the formula I in which R1, R2, R3 and R4 are each CH3 and R5 and R6 together form a ketal group of the formula Oxe2x80x94CHR13CHR14xe2x80x94O or Oxe2x80x94CH2CR15R16xe2x80x94CH2xe2x80x94O, where R13, R14, R15 and R16 are each, independently, hydrogen or C1-C3-alkyl, or the radicals R5 and R6 together represent xe2x95x90O.
The nitroxyl compound is most preferably a compound of the formula I in which R1, R2, R3 and R4 are each CH3, R5 is hydrogen, and R6 is hydrogen, OH, OR7, or Oxe2x80x94COR7, where R7 is a linear or branched saturated C1-C12-alkyl radical or an aryl or benzyl radical, xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2)nxe2x80x94OR8, xe2x80x94(Oxe2x80x94C3H6)nxe2x80x94OR8, xe2x80x94(Oxe2x80x94(CH2)4)nxe2x80x94OR8, or xe2x80x94Oxe2x80x94CH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94(Oxe2x80x94CH2xe2x80x94CH2xe2x80x94)nxe2x80x94OR8, where n=1 to 50 and R8 is hydrogen, CH2xe2x80x94CHOHxe2x80x94CH3, or CH2xe2x80x94CHOHxe2x80x94CH2xe2x80x94CH3, NR9R10, or NHCOR10, where R9 and R10 are each, independently, a linear or branched saturated C1-C12-alkyl radical or an aryl or benzyl radical.
Examples of nitroxyl compounds which can be used are TEMPO, 4-hydroxy-TEMPO, 4-oxo-TEMPO, 4-amino-TEMPO, 4-acetamido-TEMPO, 4-benzoyloxy-TEMPO, 4-acetoxy-TEMPO, and PIPO.
The nitroxyl compound is preferably used in an amount of from 0.01 to 50 mol %, most preferably in an amount of from 0.1 to 20 mol %, based on the amount of alcohol to be oxidized. It can be used as a solution in the organic phase or in the aqueous phase or in supported form as a separate phase.
As oxidant, use is made of a compound selected from the group consisting of chlorine, bromine, iodine, hypochlorite, chlorite, chlorate, hypobromite, bromite, bromate, hypoiodite, iodite, iodate, periodate, Fe(CN)63xe2x88x92, N-chloro compounds, and mixtures thereof. Preferred oxidants are hypochlorite and hypobromite. In the case of salt-like oxidants, preference is given to those having sodium, potassium, calcium, ammonium, or tetra-alkylammonium counterions.
The oxidant can also be generated in situ, e.g. electrochemically; by hydrolysis, e.g. by hydrolysis of N-chloro compounds; or by redox reactions, for example hypochlorite or hypobromite solutions obtained by disproportionation of chlorine or bromine in alkaline solution or the redox reaction between hypochlorite and bromide which leads to the formation of hypobromite.
The oxidants employed are preferably used in concentrations of from 0.1 M to their respective saturation concentration.
Further possible additives are halogens, e.g. bromine, and salts, e.g. alkali metal, alkaline earth metal or ammonium halides or sulfates, carbonates, hydrogencarbonates, phosphoric acid and its alkali metal, alkaline earth metal or ammonium salts or carbon dioxide. These additives can in each case be added to one of the two phases, if desired in solution, or may be added in dissolved form as third component to the continuous process.
In oxidations using hypochlorite, the addition of bromine or bromide in amounts of from 0.01 to 100 mol %, based on the amount of hypochlorite used, is preferred. Particular preference is given to adding bromine or bromide in amounts of from 1 to 50 mol %.
The pH of the aqueous phase is preferably in the range from 6 to 14, most preferably from 7 to 12. The desired pH is preferably established by addition of a buffer, e.g. sodium hydrogencarbonate, disodium hydrogenphosphate or sodium dihydrogenphosphate, by addition of an acid, e.g. carbon dioxide, phosphoric acid, hydrochloric acid or sulfuric acid, or a base, e.g. NaOH.
The reaction temperature is preferably from xe2x88x9210 to +80xc2x0 C., most preferably from xe2x88x925xc2x0 C. to +30xc2x0 C.
Preferred alcohols for the oxidation are all primary and secondary alcohols which have a miscibility gap with the aqueous phase, the miscibility gap established, if necessary, by addition of an organic solvent.
Primary alcohols are oxidized to aldehydes and secondary alcohols to ketones. In the case of compounds which contain both primary and secondary hydroxy groups, the primary hydroxy groups are preferentially oxidized. An overview of the order of reactivity when a plurality of hydroxy groups are present is given by A. E. J. de Nooy, A. C. Besemer and H. V. Bekkum, Synthesis 1996, 1153.
The alcohols to be oxidized are preferably linear or branched saturated or unsaturated aliphatic alcohols having 1-60 carbon atoms, cyclic aliphatic saturated or unsaturated alcohols having 5-60 carbon atoms or alcohols having 6-60 carbon atoms which are substituted by an aromatic radical, in each of which one or more hydrogen atoms or one or more carbon atoms may be replaced by heteroatoms.
Particular preference is given to linear or branched saturated or unsaturated aliphatic alcohols having 1-30 carbon atoms, cyclic aliphatic saturated or unsaturated alcohols having 5-30 carbon atoms or alcohols having 6-30 carbon atoms which are substituted by an aromatic radical, in each of which one or more hydrogen atoms may be replaced, independently of one another, by F, Cl, Br, I, NO2, ONO, CN, NC, or SCN, one or more CH2 groups may be replaced, independently of one another, by O, NH, Cxe2x95x90O, CO2, S, Sxe2x95x90O, SO2, Pxe2x95x90O, PO2, or an acetylenic C2 unit, one or more CH groups may be replaced, independently of one another, by N, B or P, and quaternary carbons may be replaced by Si, Sn or Pb.
Great preference is given to linear or branched saturated or unsaturated aliphatic alcohols having 1-30 carbon atoms, cyclic aliphatic saturated or unsaturated alcohols having 5-30 carbon atoms, or alcohols having 6-30 carbon atoms which are substituted by an aromatic radical, in each of which one or more hydrogen atoms may be replaced, independently of one another, by F, Cl, Br, I, NO2, ONO, CN, NC, or SCN, one or more CH2 groups may be replaced, independently of one another, by O, NH, Cxe2x95x90O, CO2, S, Sxe2x95x90O, SO2, Pxe2x95x90O, PO2 or an acetylenic C2 unit, one or more CH groups may be replaced, independently of one another, by N, B or P, and quaternary carbon atoms may be replaced by Si, Sn or Pb, where at least one of these substituents is present in the 2 or 3 position relative to the carbon atom bearing the hydroxy group.
Great preference is likewise given to alcohols whose partition ratio between the organic phase and the aqueous phase is at least 80:20.
Preferably, the following types of hydroxy compounds are oxidized: 2- and 3-hydroxycarboxylic esters, 2- and 3-hydroxy-carboxamides, 2-hydroxylalkyl carboxylates, N-(2-hydroxyalkyl)carboxamides, 1- or 2-hydroxyketones or dihydroxyketones, hydroxymethyl or hydroxyethyl epoxides, O-alkyl-Oxe2x80x2-2-hydroxyalkyl carbonates, bis(2-hydroxyalkyl) carbonates, 2- and 3-hydroxylactones, hydroxy-1,3-dioxolanes, hydroxy-1,3-dioxanes, hydroxy-1,4-dioxanes, 2- and 3-hydroxyimides, 2- and 3-hydroxy-alkylimides, 2- and 3-hydroxylactams, 2- and 3-hydroxy-alkylurethanes, 2-hydroxy ethers, 2,2xe2x80x2-dihydroxy ethers of oligoethylene glycols, oligopropylene glycols, 2-hydroxyalkyl alkylsulfonates or arylsulfonates, alkyl 2-hydroxyalkylsulfonates, 2-aminoalcohols, 2-trialkylsilylalcohols and 2-trialkylsilyloxyalcohols.
Examples of the above-mentioned classes of compounds are: 2-hydroxyethyl isobutyrate, 3-hydroxypropyl isobutyrate, 2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane, cyclohexylideneglycerol, pantolactone, dihydroxyacetone, glycerol, 1,4-butanediol, trimethylsilylethanol, tert-butyldimethylsilyloxyethanol, and phthalimidoethanol.
2-n-Butyryloxyacetaldehyde was obtained from 2-hydroxyethyl butyrate in an 80 percent yield at 100% conversion by means of the process of the invention.
The process of the invention likewise makes it possible to synthesize unstable substances such as trimethylsilylacetaldehyde whose preparation has previously been unsuccessful.
The following examples serve to illustrate the invention, but do not serve to limit its scope in any way.